Fighters from the militant Shiite Muslim group Hezbollah joined forces with commandos from the Lebanese army Monday to battle followers of an extremist Sunni Muslim cleric in a day of urban warfare that offered worrisome evidence that Syria’s civil war had spilled across Lebanon’s borders.

At least 18 Lebanese soldiers died in the fighting, which began Sunday. At least four Hezbollah fighters died and at least 20 gunmen from the other side were killed.

Hezbollah fighters said that in addition to Lebanese supporters of Sheikh Ahmad al Assir, who’s a staunch opponent of Hezbollah and the regime of Syrian President Bashar Assad, they’d battled radical Islamists from a nearby Palestinian refugee camp and suspected rebels from neighboring Syria.

One Hezbollah fighter said he’d killed Palestinians and Syrians in Assir’s compound.

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“That sniper we just killed had a Syrian ID card,” he called out to his commander, without noticing that two Western journalists were seated nearby. “It said he was from Aleppo.”

By the end of the fighting, Hezbollah fighters and Lebanese army special forces had taken control of Assir’s mosque complex. Dozens of Assir’s followers had been killed or captured in what participants described as bloody house-to-house fighting in a dense urban environment.

While most of Assir’s followers appeared to have died or surrendered, the cleric’s fate was unknown. Rumors of his death, injury or escape made the rounds of local news media.

Such open cooperation between the army and Hezbollah seemed likely to inflame Sunni partisans in Lebanon, where the army is seen as a neutral arbiter among the country’s religious sects. But after sustaining heavy casualties on the first day of the fighting, the army may have had little choice but to accept a massive influx of Hezbollah’s highly trained and well-equipped fighters.

“If it wasn’t for Hezbollah, this would have been a massacre of the Lebanese army,” one Hezbollah fighter said. “They would have lost more than 100 soldiers if we were not there to show them how to do this.”

Lebanese officers called Hezbollah’s intervention “limited assistance” but Hezbollah fighters described their role as crucial to vanquishing Assir’s supporters.

Speaking to two Western journalists, the top Hezbollah official on the scene – who was introduced with the Muslim honorific “Hajj” – said the group felt obligated to enter the battle because Assir had threatened Shiites and Hezbollah members in the area while promoting a political and religious agenda in line with the al Qaida-linked rebels Hezbollah has been battling in Syria.

“We don’t like having to do this,” Hajj explained while giving orders to dozens of fighters armed with sniper rifles, automatic weapons, grenade launchers and even, in one case, a truck-mounted anti-aircraft gun.

“We are forced to remove this cancerous gland from Lebanon,” he said. “But this medicine for us is bitter because we don’t wish to fight the Lebanese and Arabs. If we were fighting the Israelis, the medicine would taste sweet but instead it tastes bitter.”

The fighting began Sunday when Assir’s followers accused the army of harassing them and, in retaliation, attacked an army checkpoint in the Abra neighborhood of the port city of Sidon, which lies less than an hour south of Beirut.

The usually quiet city was wracked by two days of artillery shelling, heavy sniper fire and fierce gun battles that spread to include the nearby Ein el Hilweh refugee camp and left much of Abra in ruins.

An army statement said the offensive would continue until Sidon returned to government control, and intermittent gunfire could be heard into the night as operations – which a Hezbollah commander described as “mopping up” – continued, albeit at a much slower pace than earlier in the day.

The fighting was surprisingly fierce, considering that Assir’s followers were thought to have limited military skills. One Islamist leader from Sidon said the group had been stockpiling arms for the past two months with the support of Sunni Persian Gulf nations, which also have been arming the anti-Assad rebels in Syria.

“His men have been storing weapons for two months with money given to them by Qatar,” said Sheikh Maher Hammoud, a conservative Sunni cleric who has a close relationship with Shiite Hezbollah. Hammoud said a few dozen radical Sunni Islamists from the Palestinian camp had bolstered Assir’s military capability.

Hezbollah, which openly sent thousands of men to support the Syrian regime’s efforts to retake the rebel-held city of Qusayr over the border, was quiet about its involvement in the fighting in Sidon, but several of its men said they’d taken the lead in Monday’s battles.

“We know how to fight these sorts of battles,” Hajj said. “The Lebanese army does not have the same experience we have.”

As for the U.S. position that Hezbollah is a terrorist organization, Hajj was critical of any comparison between his Shiite group and the radical Sunni ideology of al Qaida.

“We see these evil men all over the world. They even cut the head off a soldier in England, because they’re animals who follow the wrong idea of Islam,” he said. “No Christian could have this sort of evil in their heart, only these types of men.”